It couldn’t have been a worse start in Portland. Less than 10 minutes had passed and already Seattle Sounders striker Raúl Ruidiaz finished a one-time shot over the outstretched arms of goalkeeper Jeff Attinella. Since signing with the Sounders over the summer, Ruidiaz has been sensational, carrying the Sounders to the best half-season run in MLS history—and it looked like he would be behind another rout in Portland—amid the playoffs, no less.

But that goal was “a wakeup call,” according to Timbers coach Giovanni Savarese. The Timbers clawed back to a 2-1 win in the first leg of the Western Conference semifinal—only the second time all year the Timbers conceded first and still won. It was an elusive response in an up-and-down Timbers season, and exactly the confidence boost players need if they want to win an MLS Cup.

“It’s a mental lift to know we can face adversity early on and come out on top,” said striker Jeremy Ebobisse. “That gives...